In silica-loaded tires, bis-triethoxysilyltetrasulfide is widely used as a coupling agent between rubber and silica. However, when mixed with rubber and silica at elevated temperatures, this compound acts to increase the viscosity of the blend, which is inconvenient to subsequent operation.
To overcome this problem, shorter chain polysulfide compounds such as bis-triethoxysilylpropyldisulfide were proposed. For example, JP-A 9-169774 discloses a method for preparing disulfide silanes using NaCN. This method, however, has the problem of using the toxic compound. It would be desirable to have a substitute safe method of preparing short sulfide chain-bearing organosilicon compounds at low cost.
The inventors proposed in JP-A 11-100388 a method of preparing a short sulfide chain-bearing organosilicon compound by reacting a polysulfide silane of the general formula: (RO)3SiC3H6SxC3H6Si(OR)3 wherein R is methyl or ethyl, and x is a positive number of 3 to 6 on the average, at least one anhydrous sulfur compound: M12S or M2S wherein M1 is an alkali metal or ammonium and M2 is an alkaline earth metal or zinc, and a halogenoalkoxysilane of the general formula: XC3H6Si(OR)3 wherein X is halogen and R is methyl or ethyl. When the short sulfide chain-bearing organosilicon compound is prepared by this method, however, there can also be produced a monosulfide chain-bearing organosilicon compound, that is, an organosilicon compound having a sulfide chain which does not fully participate in the reactions with silica and rubber.
The inventors further proposed in JP-A 2003-261580 (US 2003-0176719 and EP 1342750A) a method for preparing a sulfide chain-bearing organosilicon compound, comprising the steps of:
premixing a sulfide chain-bearing organosilicon compound having the general formula (i):(R1O)(3-p)(R2)pSi—R3—Sm—R3—Si(OR1)(3-p)(R2)p  (i)wherein R1 and R2 each are a monovalent hydrocarbon group of 1 to 4 carbon atoms, R3 is a divalent hydrocarbon group of 1 to 10 carbon atoms, m has an average value of 2<m≦6, and p is 0, 1 or 2, a halogenoalkyl group-bearing organosilicon compound having the general formula (ii):(R1O)(3-p)(R2)pSi—R3—X  (ii)wherein R1 and R2 each are a monovalent hydrocarbon group of 1 to 4 carbon atoms, R3 is a divalent hydrocarbon group of 1 to 10 carbon atoms, X is a halogen atom, and p is 0, 1 or 2, and optionally, sulfur,
adding anhydrous sodium sulfide represented by Na2S to the premix, and
allowing reaction to take place for thereby forming a sulfide chain-bearing organosilicon compound having the general formula (iii):(R1O)(3-p)(R2)pSi—R3—Sn—R3—Si(OR1)(3-p)(R2)p  (iii)wherein R1 and R2 each are a monovalent hydrocarbon group of 1 to 4 carbon atoms, R3 is a divalent hydrocarbon group of 1 to 10 carbon atoms, n has an average value of 2≦n<6, satisfying m>n, and p is 0, 1 or 2, while minimizing formation of a monosulfide-bearing organosilicon compound with n=1.
This method, however, has some drawbacks. While substantially anhydrous sodium sulfide is used, it is time consuming to dry sodium sulfide hydrate. Filtration of the salt is necessary. A reaction medium is generally used and must be distilled off at the end of reaction. There still exists a need for a lower cost preparation method.
It is also known to prepare sulfide chain-bearing organosilicon compounds using phase transfer catalysts. This is taught in U.S. Pat. Nos. 5,405,985, 5,468,893, 5,483,245, 6,448,426, JP-A 2004-521945 and JP-A 2004-521946. Although these patents relate to methods for preparing sulfide chain-bearing organosilicon compounds using phase transfer catalysts, no reference is made to the control of formation of a monosulfide chain-bearing organosilicon compound.